The present invention relates to a method for mooring a ship, in the open sea, especially an oil/gas production ship positioned above a submarine well, comprising three or more mooring chains/anchor lines extending from a turret disposed within the ship to anchors/anchoring points on the seabed.
Likewise, the invention relates to a mooring arrangement based on the use of said three or more mooring chains/anchor lines, and aims at achieving considerable simplifications and improvements in or indirectly relating to said turret, said simplifications manifesting themselves in significant savings, both economical and operational upon the establishment of the mooring.
Said turret has been disposed in association with a vertically through-going cavity in the ship, said cavity being formed i.a. for passage of a production riser string and/or other production accessory and, possibly, for the accommodation and temporarily attachment of a so-called STL buoy. The turret which has a substantially vertical axis, is freely rotatably mounted in the ship, for relative turning around said axis, so that the ship may turn dependent on wind and current conditions about the axis of the turret. The turret will be equipped with at least one pipeline portion which is connected to an underlying portion and which, uppermost, through a swivel device, is connected to a pipeline connected to the ship and may lead to its storage room for oil.
It is previously known to moor such a production ship to the seabed, so that it may turn about the axis of the turret.
First, according to previously known technique of this kind, e.g. eight anchor lines in the form of mooring chains or wires are positioned on the seabed and attached thereto through anchors or through other fastening by means of piles or the like, in order to, thereupon, to be drawn up via the free end portion through the turret. In known turrets of this kind, lowermost in the turret, is for each mooring chain disposed a rotary guide pulley which, normally, will be situated below the sea surface and, therefore, represents a considerable maintenance problem. For each anchor line, besides the guide pulley, at a higher situated level, a separate winch has been disposed, i.e. totally eight individual winches, each with a guide pulley assigned thereto. The winches serves to stretch and tension the individual mooring chain associated therewith, and these chains are locked in the tightened condition thereof.
This known mooring arrangement is expensive in purchase and operation, and it is relatively awkward in use.